1. Field of the Invention
This invention refers to public telephone sets using debit cards having inductive cells of the kind described in patent documents BR 7804885, BR 2558917, BR 8805894, BR 8801921 and BR 9203968.
2. Description of the Related Art
Reading and recording devices for such cells have been thoroughly studied, having in mind the decrease of manufacturing costs, increased dependability as well as size reduction of public telephone sets.
The inventions described in patent applications BR 9203968 and BR 9204434 seek to achieve such aims through a reduction in the number of electronic components used in the sensor driver circuits. Accordingly, the second of these documents describes the replacement of one of the semi-planes of inductive sensors by a slab of high permeability magnetic material, a procedure which not only halves the quantity of coils, but also eliminates the serial interconnecting elements which link each sensor coil to its correspondent opposite semi-plane. Furthermore, the number of soldered connections has been decreased and the connectors needed for said connections have been eliminated, as shown in FIG. 3 of said patent application.
The fabrication of the circuits described in the mentioned documents by means of discrete components requires specialized workmanship which burdens its cost. Furthermore, fitting the individual components one by one results in a lack of uniformity among the devices, besides shortening their life span, which should be as long as possible in hardware designed to operate for long periods under adverse conditions, such as in the case of public telephones. Moreover, esthetic reasons require a reduction in the volume of public telephone sets.
The techniques of large scale integration, on the other hand, are based on automated techniques of manufacturing and device testing, ensuring an almost total uniformity at a cost many times lower than conventional circuitry, besides bringing a substantial reduction of its physical dimensions.
However, the limitations of some components turned out by the integration technique prevent the mere transposition of standard circuit configurations used in conventional assemblies. This is the case, for instance, of the Colpitts sine wave oscillators shown in FIGS. 5 and 6 of document PI 9203968, since the application in question requires features of combined power and cut-off frequency which are beyond the capabilities of monolithic integrated devices fabricated with current production techniques.
On the other hand, the use of oscillators operating at pulsed modes, as described in document PI 8901590 may give rise to misreadings of the cell condition. This is due to the fact that the discrimination between an inductive sensor loaded with a short circuited cell and another loaded with a burned out cell is improved when such sensors are connected in parallel with capacitors so as to make up parallel resonant, or tank, circuits. Due to the variations of the inductance and capacitance values of commercially available components, the resonance frequencies of such tank circuits in a given reading/recording device are not exactly the same, bringing about a lack of coincidence between those circuits and the excitation current generated by the oscillator.